1. Field of the Disclosure
The present invention relates to a device and a method for determining the focus position of a laser beam in an ophthalmological laser projection system. The present invention more particularly relates to a device and a method for determining the focus position in the projection direction of the laser beam.
2. Related Art
In ophthalmology, pulsed laser beams, for example laser pulses with a pulse duration in the femtosecond range (1 fs=10−15 s), are used for treating eye tissue. In the process, incisions are carried out by focused projection of the laser pulses into the eye tissue. As a result of production tolerances and/or thermal expansions during the production and operation of ophthalmological laser projection systems, the precise focus position of the laser pulses in the projection direction is generally not known with a sufficiently high accuracy with respect to the ophthalmological laser projection system, more particularly with respect to the projection lens and/or a contact body through which the laser pulses are projected into the tissue. Even if the focus position can be determined by calibration during the production of the laser projection system, it is possible for this position to change during operation, for example in the case of thermal material expansions or when replacing the utilized contact body.
EP 2 069 099 describes a method for confocal detection of the entry surface and exit surface of a contact body. As per EP 2 069 099, a laser measurement beam is preferably focused onto the surface by means of a variable focus adjustment apparatus and returned or reflected radiation is detected confocally. The position of the surfaces is determined from the confocally detected radiation and the associated setting of the variable focus adjustment apparatus. The method as per EP 2 069 099 requires a focusing movement for determining the surfaces, which can adversely affect the measurement accuracy. Moreover, the confocal detection supplies very weak signals, and therefore much metrological effort is required for accurate measurements.